1. Field of the Invention
The present invention relates to a method for concentrating a solution, and more especially a method for concentrating a solution or a polymer dispersion solution which is used for producing an optical film provided in an optical product.
2. Description Related to the Prior Art
In order to produce a polymer film, such as a cellulose acylate film and the like, polymers are dissolved in a solvent to prepare a low concentration dope, namely, a polymer solution or a polymer dispersion solution having low concentration. The low concentration dope has lower concentration than a high concentration dope which is used for forming a polymer film. Afler preparation, the low concentration dope is oflen concentrated to the high concentration dope. The high concentration dope is used for forming the polymer film in a method for producing the polymer film to obtain it. Thus, the time for dissolving the solute becomes shorter, and it is prevented that the undissolved material remains in the low concentration dope. As a method of concentrating the low concentration dope, there are methods of flash evaporation and a thin film evaporation. In the method of flash evaporation, especially of spray flash evaporation disclosed in U.S. Pat. No. 4,414,341, the heated low concentration dope is fed out from a nozzle (“orifice”) in a tank, and thereby forms tiny droplets such that the total size of surfaces of the tiny droplets becomes larger. Accordingly, the solvent in the low concentration dope can be easily evaporated.
As shown in FIG. 7, in a method of the spray flash evaporation in the prior art, for example, a concentrating apparatus 140 is used. The concentrating apparatus includes a tank main body 141 and a jacket 142 for controlling a temperature of the tank main body 141, and a medium 143 flows in a space between the tank main body 141 and the jacket 142. In an upper side, the tank main body 141 has a nozzle 148 for performing a flash evaporation of a low concentration dope 147 which has been filtrated with a filtration apparatus 152. An end 148a of the nozzle 148 is positioned over a surface 144a of a concentrating dope 144, which is obtained from the low concentration dope 141 through flash evaporation and contained in the tank main body 141. In the tank main body 147, there is a stirrer 145 for stirring the concentrating dope 144. The stirrer 145 is connected with a motor 146 and controlled a rotation thereof Further, a condenser 149 is connected through a pipe 150 to the tank main body 141. In the tank main body 141, a solvent of the concentrating dope 144 is evaporated to become a gas solvent 144b, and the gas solvent 144b is condensed with the condenser 149. Thereby, the concentrating dope 144 is concentrated to a high concentration dope 151, and the high concentration dope 151 is fed out and filtrated with a filtration apparatus 153 to use for forming the polymer film.
However, when the low concentration dope 147 is fed out from the nozzle 148, tiny droplets 147a of the low concentration dope 147 are sprayed, and some of the tiny droplets 147a are adhered on an inner wall 141a of the tank main body 141 to generate a skinning 147b, easily. The skinning 147b varies the composition of the high concentration dope 151 and pollutes the inside of the tank main body 141. The skinning 147b is removed from the high concentration dope 151 by the filtration apparatus 153, and when the filtration apparatus 153 is stopped with the skinning, the filtration apparatus 153 is exchanged for another one. In this case, time for exchange of the high concentration dope 151 is necessary, and flow rate varies, which prevents to continuously obtain the high concentration dope 151 having a uniform concentration. Further, a little of the high concentration dope 151 remains in the removed filtration apparatus 153 at exchange. Furthermore, the removed filtration apparatus 151 is cleansed with solvent. Accordingly, cost therefore cannot be reduced.
Further, in order to prevent the concentrating dope 144 from drying, a temperature of the inner wall 141a is lower than the boiling point of the concentrating dope 144. However, in the method of flash evaporation in the prior art, when the temperature of the inner wall 141a is lower than the boiling point, the solvent gas 144b is condensed on the inner surface 141a, and the condensate is oflen mixed with the concentrating dope 144. Thus the concentrating dope 144 is diluted, and the efficiency of concentrating the low concentration dope 147 becomes lower. Further, in order to evaporate the solvent uniformly, the concentrating apparatus 140 must have such a large space (whose capacity is from 200 L to 10000 L) that the dope surface 144a is enough large and the tiny droplets 147a are hardly adhered to the inner wall 141a, while the stirrer 145 stirs the concentrating dope 144 . Therefore it is hard to secure a space for setting the concentrating apparatus 140. Furthermore, as the capacity of the concentrating apparatus 144 is large, the concentrating dope 144 remains in the tank main body 145 for from one to four hours. Accordingly, the high concentration dope 151 is sometimes denaturated.
Furthermore, as the low concentration dope is sprayed in a gas phase, gas is dissolved therein, and the concentrating dope 144 contains a larger amount of gas. When the high concentration dope prepared from such concentrating dope 144 is cast dried on a band, gas is generated, which prevents the formation of a polymer film having a smooth surface. Accordingly, degassing from the high concentration dope is necessary before use for forming the polymer film.